Metal halide discharge lamp having heat reflective coating

ABSTRACT

The arc tube ends of a metal halide arc discharge lamp have a heat reflective coating thereon. The coating comprises zirconium dioxide and zirconium diboride.

United States Patent Keeffe June 10, 1975 METAL HALIDE DISCHARGE LAMP[56] References Cited HAVING HEAT REFLECTIVE COATING UNITED STATESPATENTS Inventor: William Keeffe, Rockport, Mass. 3,662,203 5/1972 Kuhlet a1. 313/221 [73] Assignee: GTE Sylvania Incorporated,

Danvers, M Primary ExaminerJames B. Mullins Attorney, Agent, orFirm-James Theodosopoulos [22] Flledz Mar. 25, 1974 [21] Appl. No.:454,397 5 ABSTRACT The are tube ends of a metal halide arc discharge US.Cll 3 I 3 lamp havg a heat reflective coating thereon. The oat- [5 Int.Cl. omprises irconium dioxide; and irconium dibo- [58] Field of Search313/17, 27, 44, 47, 220, i

4 Claims, 1 Drawing Figure METAL HALIDE DISCHARGE LAMP HAVING HEATREFLECTIVE COATING THE INVENTION This invention relates to metal halidearc discharge lamps. Such lamps comprise an arc tube, usually made offused quartz or other high silica glass, having electrodes disposedtherewithin and containing a fill including an inert starting gas,mercury and a metal halide.

The are tube ends of such lamps usually have a heat reflective coatingthereon in order to maintain said ends at a sufficiently hightemperature so as to ensure adequate vapor pressure of the metal halidesin the arc tube. Examples of such coatings are shown in US. Pat. Nos.3,325,662 and 3,374,377, which disclose coatings of calciumpyrophosphate and zirconium dioxide.

One of the problems with prior art coatings is lack of adhesion to thearc tube. This problem is not severe when the arc tube is enclosedwithin an outer jacket. However, in unjacketed lamps, the rubbing-off ofthe coating can be detrimental to proper lamp operation.

This invention concerns zirconium dioxide heatrefiective coatings havingimproved adhesion and improved resistance to abrasion. l have found thata coating comprising ZrO and ZrB which has been fired at a suitably hightemperature, yields said improvements.

The single FIGURE in the drawing is an elevational view, partly insection, of the arc tube of a metal halide discharge lamp in accordancewith this invention.

The are tube comprises a tubular quartz envelope 1 having press seals 2at each end thereof. Disposed within envelope 1 is the usual fillincluding an inert starting gas, mercury and metal halide. An electrode3 is disposed at each end of the arc tube, each electrode beingsupported on metal rod 4 which is welded to molybdenum ribbon 5 wich, inturn, is welded to external lead-in wire 6. Press seal 2 completelyembeds ribbon 5 and provides the support for rod 4.

Disposed on the exterior surface of the cup-shaped ends of envelope 1 isa heat reflecting coating 7. Coating 7 generally surrounds electrode 3and extends onto press seal 2. The coating comprises ZrO and ZrB theaddition of ZrB considerably improving the adhesion of the coating tothe quartz.

In a specific example, 125 grams of powdered ZrO 3 grams of powdered ZrBand 3.2 grams of submicron powdered alumina were dispersed in 200milliliters of isopropyl alcohol. The specific gravity of this coatingsuspension was about 1.9. Each end of sealed envelope 1 was dipped intothe suspension up to about or slightly beyond the height of electrode 3to form coating 7 thereon. The coating was then removed from the end ofpress seal 2 and then fired at a temperature of 550 to 800C in order toimprove the adhesion thereof.

X-ray diffraction analysis of the bonded coating revealed a drop in ZrBintensity and an increase in ZrO intensity in comparison with theunfired coating. This suggests that the ZrB is being reduced and thatthe Zr metal thereby freed up is oxidized during the firing, therebyincreasing the Zr0 concentration while reducing the ZrB concentration.The boron thus formed goes into a borosilicate glassy phase of thegeneral form (x)SiO (xa)B O at the quartz interface which plays a rolein the bonding mechanism. Scanning electron microscope microphotographsof the interface between coating 7 and quartz envelope 1 confirms theformation of the borosilicate glassy phase.

At a suspension specific gravity of about 1.9, the thickness of coating7 was about microns, which is satisfactory for the purpose of thisinvention. If the coating is too thin, the heat reflectivity thereof isinadequate. If the coating is too thick, cracking thereof can result.

The ratio of ZrO to ZrB in the coating suspension should be betweenabout 30:1 and 250:1. When the ZrB content is at a lesser ratio thanabout 250:1 there is insufficient ZrB present to improve adhesion. Atgreater ratios than about 30:1, the ZrB discolors the coating andreduces the visible light reflectivity thereof, thereby reducing lampefficiency.

When the coating is fired at temperatures less than about 550C, theadhesion thereof to the arc tube is inadequate to prevent rub-off.

The purpose of the submicron alumina in the coating suspension is toincrease the strength of the unfired coating.

When the arc tube is supported by metal supports 8 clamped on press seal2, there should be a separation between clamp 8 and coating 7. Thereason for this is to prevent electrolysis that can occur as a result ofa potential difference between electrode 3 and coating 7, since coating7 has some electrical conductivity at normal operating temperatures andsince clamp 8 is usually a part of the electrical circuit of the lamp.

Lamps of this invention that have been life tested have shown noevidence of deleterious strains in the quartz as a consequence of thereaction of the ZrB with quartz.

I claim:

1. An arc discharge lamp comprising an arc tube, made of high silicaglass and having press seals at each end, containing a filling includinginert starting gas, mercury and metal halide, having electrodes sealedtherein at opposite ends and having a heat reflective coating of ZrO andZrB on the ends of said are tube.

2. The lamp of claim 1 wherein said coating surrounds said electrodesand extends onto said press seals.

3. The lamp of claim 1 wherein the ratio of ZrO to ZrB in said coatingis between about 30:1 and 250:1.

4. The lamp of claim 1 including, in addition, metal supports clamped onsaid press seals and an uncoated space between said metal supports andsaid heat reflective coating.

1. AN ARC DISCHARGE LAMP COMPRISING AN ARC TUBE, MADE OF HIGH SILICAGLASS AND HAVING PRESS SEALS AT EACH END, CONTAINING A FILLING INCLUDINGINERT STARTING GAS, MERCURY AND METAL HALIDE, HAVING ELECTRODES SEALEDTHEREIN AT OPPOSITE ENDS AND HAVING A HEAT REFLECTIVE COATING OF ZRO2AND ZRB2 ON THE ENDS OF SAID ARC TUBE.
 2. The lamp of claim 1 whereinsaid coating surrounds said electrodes and extends onto said pressseals.
 3. The lamp of claim 1 wherein the ratio of ZrO2 to ZrB2 in saidcoating is between about 30:1 and 250:1.
 4. The lamp of claim 1including, in addition, metal supports clamped on said press seals andan uncoated space between said metal supports and said heat reflectivecoating.